As is well known to those skilled in the art, it is desirable to increase the quantity of products boiling in the gasoline boiling range recovered from crude oil. This is commonly effected by catalytically cracking heavier cuts derived from crude oil distillation--typically a gas oil which may have an ibp of 400.degree. F.-800.degree. F., say 600.degree. F., a 50% bp of 550.degree. F.-975.degree. F., say 800.degree. F. and a 90% bp of 600.degree. F.-1035.degree. F., say 880.degree. F.
As the supply of high quality (i.e. low sulfur-containing) crudes has decreased, there is an increasing tendency to utilize as charge to cracking units, hydrocarbon streams containing undesirably high content of sulfur.
When these sulfur-containing hydrocarbons are cracked, a portion of the sulfur content is released in the reactor as mercaptans or principally as hydrogen sulfide -due to the reducing conditions prevailing in the reactor. These sulfur-containing components are recovered with the reactor overhead and may be readily removed from the liquid condensate by various processes including Doctor treating etc.
A portion of the sulfur content of the charge hydrocarbon is deposited with the coke onto the catalyst particles. Clearly the sulfur content of the spent catalyst at this point depends on the sulfur content of the charge hydrocarbon; but it is not uncommon to find spent catalyst containing as much as 0.10 w % sulfur, and typically 0.01 w %-0.06 w % sulfur.
The spent catalyst, bearing coke plus sulfur, is passed to a regenerator wherein the carbon is burned off the catalyst to yield regenerated catalyst plus a gas containing oxides of carbon. Unfortunately this gas also contains oxides of sulfur--typically sulfur dioxide and sulfur trioxide, commonly referred to as SO.sub.x. The SO.sub.x content of the regenerator off-gas may undesirably be as high as 2500 wppm, and when cracking a typical high-sulfur gas oil with sulfur content of 1.5 w %-3 w %, say 2.5 w %, it may commonly be 900 vppm-1800 vppm, say 1500 vppm.
Environmental considerations dictate that the sulfur content of the regenerator off-gas be decreased, preferably to below 500 vppm, say 100 vppm-300 vppm. There is a substantial body of prior art directed to this end. Illustrative of the many patents and literature on this subject may be noted U.S. Pat. Nos. 4,071,436; 4,432,890; WO 82/03225; and the article by Lowell et al Selection of Metal Oxides for Removing SO.sub.2 from Fuel Gas I.E.C. (Proc. Des. Dev) Vol 10, No 3 (1971) p 384-390.
The prior art generally attempts to solve the problem by preparing a catalyst which includes components which it is hoped will decrease the sulfur content of the regenerator off gas to desired low levels.
It is an object of this invention to provide a novel method for reducing the sulfur content of the regenerator off gas of a catalytic cracking unit. Other objects will be apparent to those skilled in the art.